Metal-matrix composites (MMCs) are composed of a metal and a reinforcing material distributed within the metal. The metal matrix is usually comprised of titanium (Ti) or aluminum (Al), and the reinforcing material is usually comprised of alumina (Al.sub.2 O.sub.3) or silicon carbide (SiC), in the form of particulates, whiskers, wires, fibers, or flakes. MMCs are lighter and have superior mechanical and thermal properties than the metals comprising the matrix alone, including higher specific strength, operating temperature, and wear resistance. MMCs are increasingly important in a growing number of applications, including use in the aerospace industry, high speed mechanical systems, and electronic packaging. The potential use of MMCs, however, is largely unrealized because MMC elements must be joined together to produce integrated structures, and methods for joining MMC elements by autogenous welding have been largely unsuccessful.
Welding technology has developed according to new sources of thermal energy required for directed, localized heating and melting. For example, electrical energy made available the electric arc for welding applications, with attendant improvements including gas tungsten arc (GTA), gas metal arc (GMA), and submerged-arc welding. A major problem associated with using conventional electric arc welding for joining MMCs is that chemical and metallurgical reactions occur between the metal matrix and the reinforcing material, causing brittle and excessively porous welds having poor structural strength. For example, attempts to fusion weld MMCs reinforced with SiC cause undesirable, irreversible chemical reactions rendering the joints unusable.
Electron beam welding tightly focuses an electron beam to effectively drill a hole through the thickness of the target material, permitting single pass and deep penetrating welds. In the deep penetrating welding process, a beam of relatively high velocity electrons (50 KeV to 150 KeV) is directed at the joint to be welded. The beam is focused to produce a power density of about 10.sup.6 watts/cm.sup.2 at the target surface, superheating the surface layers of the work piece and causing violent vaporization, particularly in the central region of the beam, whereby a slightly conical hole, referred to as a keyhole, is bored through the thickness of the material to be welded. The keyhole, having a diameter usually slightly less than the beam diameter, is filled with products of the fusion, evaporation, and sublimation process. Thermal interaction of the metal being welded by relative beam movement occurs at the front wall of the vapor-gas channel (the keyhole), and molten metal moves from the front wall, along the channel walls, and to the rear of the keyhole, due to the dynamic balance between the pressure of gases, vapors, and molten metal. Provided the established condition produces a stable keyhole, a high quality, full penetration welded joint is produced. A lower quality, or slightly defocused, electron beam at a specific power of about 10.sup.4 watts/cm.sup.2 behaves similarly to a GTA weld, i.e. penetration is limited by heat conductance into the thickness of the metal from the heat area on the surface.
Conditions of laser beam energy sources are similar to the electron beam sources, however, the specific energy of the laser beam must be greater than about 5.times.10.sup.6 watts/cm.sup.2 to produce a keyhole, due to lower coupling efficiency of the laser beam, as well as metallurgical reactions of the molten metal and the atmosphere. CO.sub.2 lasers are used for welding thin sheet metal components because of the high welding speed made possible by the high power of the focused laser beam. However, as the thickness of the welding joint increases, a gas plume forms from metal vaporized by the high intensity beam. The metal vapor is ionized due to laser irradiation which obstructs the beam transfer into the weld pool. A jet of helium gas must be used to blow the plume away from the work surface. Alternatively, laser beam welding is conducted in a vacuum where the metal vapor expands so rapidly that no blocking plume can form.
All energy sources currently utilized for fusion welding, including oxy-fuel, electric arc, GTA, GMA, plasma arc, electron beam, and laser energy sources, essentially depend on surface heating and the resultant conductive heat flow for penetration of the melt into the thickness of the weld. A relatively large temperature difference must be created between the face and root of the weld in order to achieve full penetration, and, therefore, the surface layer of the melt is superheated. Subjecting the surface layer of MMCs to superheating causes melting and vaporization of the reinforcement material, destroying or decreasing the strength of the MMCs. Thus, conventional fusion welding processes cause pronounced changes in the distribution of the reinforcement material of the MCCs in the solidified weld metal. For example, for an MCC reinforced with Al.sub.2 O.sub.3 material, fusion welding results in a joint strength essentially equivalent to the unreinforced aluminum matrix alloy.
A need still exists in the art for a method of directed, localized heating for welding metal-matrix composites, without excessive melting and/or vaporization.
The development of synchrotron radiation sources has made high energy x-rays available as a volumetric heat source for material processing. In contrast to conventional welding methods, the photons of the x-ray beam penetrate the complete thickness of the work piece, nearly instantaneously heating the volume through which they pass. Thus, the surface layers of the work piece are not subjected to superheating. High energy x-rays have the ability to penetrate deeply into light alloys at selected power densities, without causing violent vaporization and/or holes in the target material. The absorption coefficient of x-rays are atom and energy dependent, and therefore the behavior of the target material exposed to x-rays is vastly different than the behavior of the same material subjected to electron or laser irradiation.
The present method uses x-rays having relatively short wave lengths, such as 1-8 Angstroms, and a relatively high energy power, such as 10.sup.5 watts/cm.sup.2, to weld metal-matrix composites. A high power x-ray is directed to the weld line between two adjacent MMCs materials, generating an irradiated region or melt zone. The metal matrices are fused together in the melt zone, while the reinforcing material of the metal-matrix composites is not vaporized, but remains uniformly distributed in the melt zone. In an alternate embodiment, high power x-rays are used to provide the volumetric heat required to weld metal elements, including metal elements comprised of metal alloys.
Therefore, in view of the above, a basic object of the present invention is to provide a method for welding metal matrix composites, metals, and metal alloys.
Another object of this invention is to provide a method for welding that penetrates deep into a metal matrix composite without causing vaporization of the reinforcement material.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.